Process for isomerizing a non-equilibrium alkylaromatic feed mixture and an aromatic production facility

ABSTRACT

One exemplary embodiment can be a process for the isomerization of a non-equilibrium alkylaromatic feed mixture. The process can include contacting the non-equilibrium alkylaromatic feed mixture in a C8 isomerization zone. The C8 isomerization zone may include a first isomerization stage and a second isomerization stage. At the first isomerization stage, at least a portion of the non-equilibrium alkylaromatic feed mixture can be contacted at a first isomerization condition in a liquid phase in the substantial absence of hydrogen to obtain an intermediate stream. At the second isomerization stage, at least part of the intermediate stream and at least a part of a stream rich in at least one naphthene can be contacted at a second isomerization condition to obtain a concentration of at least one alkylaromatic isomer that is higher than a concentration of that at least one alkylaromatic isomer in the non-equilibrium feed mixture.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of application Ser. No. 11/954,462 whichwas filed Dec. 12, 2007, now allowed, the contents of which areincorporated herein by reference thereto.

FIELD OF THE INVENTION

The field of this invention generally relates to a process forisomerizing a non-equilibrium alkylaromatic feed mixture and/or anaromatic production facility that, at least in part, may isomerize anon-equilibrium alkylaromatic feed mixture.

BACKGROUND OF THE INVENTION

The xylenes, such as para-xylene, meta-xylene and ortho-xylene, can beimportant intermediates that find wide and varied application inchemical syntheses. Generally, paraxylene upon oxidation yieldsterephthalic acid that is used in the manufacture of synthetic textilefibers and resins. Meta-xylene can be used in the manufacture ofplasticizers, azo dyes, and wood preservers. Generally, ortho-xylene isa feedstock for phthalic anhydride production.

Xylene isomers from catalytic reforming or other sources generally donot match demand proportions as chemical intermediates, and furthercomprise ethylbenzene, which can be difficult to separate or to convert.Typically, para-xylene is a major chemical intermediate with significantdemand, but amounts to only 20-25% of a typical C8 aromatic stream.Adjustment of an isomer ratio to demand can be effected by combiningxylene-isomer recovery, such as adsorption for para-xylene recovery,with isomerization to yield an additional quantity of the desiredisomer. Typically, isomerization converts a non-equilibrium mixture ofthe xylene isomers that is lean in the desired xylene isomer to amixture approaching equilibrium concentrations. It is also desirable toconvert ethylbenzene to one or more xylenes while minimizing xyleneloss. Moreover, other desired aromatic products, such as benzene, can beproduced from such processes.

Various catalysts and processes have been developed to effect xyleneisomerization. In one such system, isomerization can include separatereactors having different functions. Particularly, one reactor having afirst isomerization catalyst can perform xylene isomerization with lowethylbenzene conversion, while the other reactor having a secondisomerization catalyst may perform ethylbenzene conversion with lowxylene isomerization. If the ethylbenzene reactor can selectivelyconvert ethylbenzene into one of the xylene isomers, typicallypara-xylene, then above-equilibrium levels of the preferred isomer canbe obtained. Depending on the isomerization catalyst, the ethylbenzenemay also be converted to xylenes, or may simply be dealkylated. In thisway, the desired product yield is maximized by converting the undesiredcomponents. Yield is greatest when undesired products can be minimizedand ethylbenzene conversion can be maximized.

One way to reduce loss of cyclic hydrocarbons having eight carbon atoms(hereinafter may be abbreviated as “C8 ring loss” or “C8RL”) is tooperate in a liquid phase. In the absence of hydrogen, saturation andcracking reactions may be essentially eliminated.

Generally, C8 naphthenes are intermediates for an ethylbenzeneconversion bed in the isomerization of ethylbenzene to xylenes.Typically, C8 naphthenes are not intermediates for xylene isomerization,although the C8 naphthenes may be included in the feed to the xyleneisomerization zone. Alternatively, if the ethylbenzene level is very lowor zero, only the xylene isomerization may be required.

Thus, it would be desirable to improve the operation of the liquid-phasexylene isomerization by altering the feed to reduce C8RL whileincreasing isomerization activity.

BRIEF SUMMARY OF THE INVENTION

One exemplary embodiment can be a process for the isomerization of anon-equilibrium alkylaromatic feed mixture. The process can includecontacting the non-equilibrium alkylaromatic feed mixture in a C8isomerization zone. The C8 isomerization zone may include a firstisomerization stage and a second isomerization stage. At the firstisomerization stage, at least a portion of the non-equilibriumalkylaromatic feed mixture can be contacted at a first isomerizationcondition in a liquid phase in the substantial absence of hydrogen toobtain an intermediate stream. At the second isomerization stage, atleast part of the intermediate stream and at least a part of a streamrich in at least one naphthene can be contacted at a secondisomerization condition to obtain a concentration of at least onealkylaromatic isomer that is higher than a concentration of that atleast one alkylaromatic isomer in the non-equilibrium alkylaromatic feedmixture.

Another exemplary embodiment can be a process for the isomerization of anon-equilibrium alkylaromatic feed mixture. The process can includecontacting the non-equilibrium alkylaromatic feed mixture in a C8isomerization zone. The C8 isomerization zone may include a firstisomerization stage and a second isomerization stage. At the firstisomerization stage, at least a portion of the non-equilibriumalkylaromatic feed mixture is contacted at a first isomerizationcondition in a liquid phase in the substantial absence of hydrogen toobtain an intermediate stream. At the second isomerization stage, atleast part of the intermediate stream and all of a stream comprisingsubstantially at least one naphthene may be contacted at a secondisomerization condition to obtain a concentration of at least onealkylaromatic isomer that is higher than a concentration of that atleast one alkylaromatic isomer in the non-equilibrium alkylaromatic feedmixture.

A further exemplary embodiment can be an aromatic production facility.The aromatic production facility can include a xylene isomer separationzone, a C8 aromatic isomerization zone receiving a non-equilibriumalkylaromatic feed mixture from the xylene isomer separation zone, and aseparation zone for obtaining at least part of a stream rich in at leastone naphthene. The C8 aromatic isomerization zone may further includefirst and second isomerization stages. At the first isomerization stage,at least a portion of the non-equilibrium alkylaromatic feed mixture iscontacted at a first isomerization condition in a liquid phase in thesubstantial absence of hydrogen to obtain an intermediate stream. At asecond isomerization stage, at least part of the intermediate stream andat least part of a stream rich in at least one naphthene from theseparation zone may be contacted at a second isomerization condition toobtain a concentration of at least one alkylaromatic isomer that ishigher than a concentration of that at least one alkylaromatic isomer inthe non-equilibrium alkylaromatic feed mixture.

Therefore, the process and/or aromatic production facility can improveactivity, selectivity, and stability. Particularly, the process canprovide increased isomerization of xylenes while minimizing C8RL.

DEFINITIONS

As used herein, the term “zone” can refer to an area including one ormore equipment items and/or one or more sub-zones. Equipment items caninclude one or more reactors or reactor vessels, heaters, separators,exchangers, pipes, pumps, compressors, and controllers. Additionally, anequipment item, such as a reactor or vessel, can further include one ormore zones or sub-zones.

As used herein, the term “stream” can be a stream including varioushydrocarbon molecules, such as straight-chain, branched, or cyclicalkanes, alkenes, alkadienes, and alkynes, and optionally othersubstances, such as gases, e.g., hydrogen, or impurities, such as heavymetals. The stream can also include aromatic and non-aromatichydrocarbons. Moreover, the hydrocarbon molecules may be abbreviated C1,C2, C3 . . . Cn where “n” represents the number of carbon atoms in thehydrocarbon molecule.

As used herein, the term “aromatic” can mean a group containing one ormore rings of unsaturated cyclic carbon radicals where one or more ofthe carbon radicals can be replaced by one or more non-carbon radicals.An exemplary aromatic compound is benzene having a C6 ring containingthree double bonds. Other exemplary aromatic compounds can includepara-xylene, ortho-xylene, meta-xylene and ethylbenzene. Moreover,characterizing a stream or zone as “aromatic” can imply one or moredifferent aromatic compounds.

As used herein, the term “support” generally means a molecular sievethat has been combined with a binder before the addition of one or moreadditional catalytically active components, such as a metal, or theapplication of a subsequent process such as reducing, sulfiding,calcining, or drying. However, in some instances, a support may havecatalytic properties and can be used as a “catalyst”.

As used herein, the term “non-equilibrium” generally means at least oneC8 aromatic isomer can be present in a concentration that differssubstantially from the equilibrium concentration at a differentisomerization condition.

As used herein, the term “substantially all” is intended to indicate anamount over about 90 weight percent, preferably over about 95 weightpercent, of the total amount of the referenced compound or group ofcompounds.

As used herein, the term “rich” is intended to indicate a concentrationover about 50 weight percent, preferably over about 65 weight percent,of the referenced compound or group of compounds.

As used herein, the term “substantial absence of hydrogen” means that nofree hydrogen is added to a feed mixture and that any dissolved hydrogenfrom prior processing is substantially less than about 0.05 moles/moleof feed, frequently less than about 0.01 moles/mole, and possibly notdetectable by usual analytical methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of an exemplary aromatic productionfacility.

FIG. 2 is a schematic depiction of another exemplary aromatic productionfacility.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an exemplary aromatic production facility 20 caninclude a xylene isomer separation zone 100, a C8 isomerization zone200, and a separation zone 300. The aromatic production facility 20 caninclude other zones or units, such as an alkylation, an extractivedistillation, and/or an olefin saturation zone or unit, as disclosed in,for example, U.S. Pat. No. 6,740,788 B1.

The xylene isomer, such as a para-xylene or meta-xylene, separation zone100 can receive an alkylaromatic feed mixture in a line 80. Typically,the feed mixture may be derived from any of a variety of originalsources, e.g., petroleum refining, thermal or catalytic cracking ofhydrocarbons, coking of coal, or petrochemical conversions in, e.g., arefinery or petrochemical production facility. Preferably the feedmixture is found in appropriate fractions from variouspetroleum-refinery streams, e.g., as individual components or as certainboiling-range fractions obtained by the selective fractionation anddistillation of catalytically cracked or reformed hydrocarbons.

The xylene isomer separation zone 100 can include one or more reactors120 to produce an extract of a desired isomer, such as para-xylene, in aline 110 and a raffinate in a line 114. The xylene isomer separationzone 100 may be based on a fractional crystallization process or anadsorptive separation process. An adsorptive separation process canrecover over about 99%, by weight, para-xylene in the line 110 at highrecovery per pass. An exemplary xylene isomer separation zone 100 isdisclosed in U.S. Pat. No. 6,740,788 B1. The raffinate, which is aneffluent from the zone 100, can be sent via the line 114 and a line 104to the C8 isomerization zone 200.

Typically, the raffinate substantially comprises the non-equilibriumalkylaromatic feed mixture in the line 104. The non-equilibriumalkylaromatic feed mixture can include isomerizable alkylaromatichydrocarbons of the general formula C₆H_((6-n))R_(n), where n is aninteger of 1-5 and R is CH₃, C₂H₅, C₃H₇, or C₄H₉, in any combinationsuitable for isomerization to obtain at least one more valuablealkylaromatic isomer, such as para-xylene or meta-xylene, in anisomerized product. The feed mixture can include one or moreethylaromatic hydrocarbons containing at least one ethyl group, i.e., atleast one R of at least one of the alkylaromatic hydrocarbons is C₂H₅.Suitable components of the feed mixture generally include, for example,an ethylbenzene, a meta-xylene, an ortho-xylene, a paraxylene, anethyl-toluene, a trimethylbenzene, a diethyl-benzene, a triethylbenzene,a methylpropylbenzene, an ethylpropylbenzene, a diisopropylbenzene, or amixture thereof. Typically, the one or more ethylaromatic hydrocarbonsare present in the feed mixture in a concentration of about 2-about100%, by weight.

Usually the non-equilibrium mixture is prepared by removal of para-,ortho- and/or meta-xylene from a fresh C8 aromatic mixture obtained fromone or more aromatic-production or aromatic-conversion processes toyield a stream depleted in at least one xylene isomer. Generally,isomerization of a non-equilibrium C8 aromatic feed mixture includingxylenes and ethylbenzene to yield para-xylene is a particularlypreferred application. Typically, such a mixture may have anethylbenzene content in the approximate range of about 1-about 50%, byweight, an ortho-xylene content in the approximate range of about0-about 35%, by weight, a meta-xylene content in the approximate rangeof about 0-about 95%, by weight, and a para-xylene content in theapproximate range of about 0-about 30%, by weight.

The alkylaromatic-containing streams such as catalytic reformate with orwithout subsequent aromatic extraction can be isomerized to producespecified xylene isomers and particularly to produce para-xylene. A C8aromatic feed may contain non-aromatic hydrocarbons, i.e., naphthenesand paraffins, in an amount up to about 30%, by weight. Preferably theisomerizable hydrocarbons consist essentially of aromatics, however, toensure pure products from downstream recovery processes. Typically, thenon-equilibrium alkylaromatic feed mixture is an effluent from a xyleneisomer separation zone.

Accordingly, an alkylaromatic hydrocarbon feed mixture may be contactedsequentially with two or more catalysts respectively in a C8isomerization zone 200 having first and second isomerization stages 220and 260. Typically, the first isomerization stage 220 is at least forisomerizing at least one xylene and the second isomerization stage 260is at least for isomerizing ethylbenzene. However, it should beunderstood that other reactions can occur in addition to isomerizing atleast one xylene and ethylbenzene in respective stages 220 and 260.Contacting may be effected in either stage 220 or 260 using a fixed-bedsystem, a moving-bed system, a fluidized-bed system, a slurry system oran ebullated-bed system, or a batch-type operation. Preferably, afixed-bed system is utilized in both stages 220 and 260. Each stage 220and 260 can include, respectively, at least one reactor 230 and at leastone reactor 290.

In a preferred manner, the feed mixture is preheated by suitable heatingmeans as known in the art to the desired reaction temperature and passesin a liquid phase in the substantial absence of hydrogen into the firstisomerization stage 220 containing at least one fixed reactor 230 havinga first isomerization catalyst. The first isomerization stage 220 mayinclude a single reactor 230, or two or more separate reactors withsuitable measures to ensure that the desired isomerization temperatureis maintained at the entrance of each reactor. The reactants may becontacted with the catalyst bed in upward-, downward-, or radial-flowfashion to obtain an intermediate stream that may contain alkylaromaticisomers in a ratio differing from the feed mixture. In the preferredprocessing of one or more C8 aromatics, the intermediate stream cancontain xylenes in proportions closer to equilibrium than in the feedmixture plus ethylbenzene in a proportion relating to the feed mixture.

Generally, the first isomerization catalyst includes a molecular sieve,such as a zeolite. The zeolite may contain one or more metals, such asgallium and/or aluminum. Typically, the zeolite is an aluminosilicatezeolite, having a Si:Al₂ ratio greater than about 10, preferably greaterthan about 20, and a pore diameter of about 5-about 8 angstroms (Å).Specific examples of suitable zeolites are MFI, MEL, EUO, FER, MFS, MTT,MTW, TON, MOR, and FAU zeolites, as disclosed in US 2007/0004947. Oneexemplary MFI-type zeolite is a gallium-aluminum-MFI, with gallium andaluminum as components of the crystal structure.

The preparation of a zeolite by crystallizing a mixture including agallium source, optionally an aluminum source, a silica source, andoptionally an alkali metal source is known. Conversion of analkali-metal-form zeolite to the hydrogen form may be performed bytreatment with an aqueous solution of a mineral acid. Alternatively,hydrogen ions can be incorporated into the pentasil zeolite by ionexchange with ammonium salts such as ammonium hydroxide or ammoniumnitrate followed by calcination. Desirably, an aluminosilicate zeolitecan contain at least about 40%, and preferably about 40-about 46%, byweight, silicon, calculated on an elemental basis based on the molecularsieve. In addition, the aluminosilicate zeolite can contain generallyabout 0.5-about 7.0%, desirably about 2.0-about 5.0%, and optimallyabout 2.5-about 3.5%, by weight, gallium, calculated on an elementalbasis based on the molecular sieve. Furthermore, the aluminosilicatezeolite can contain generally about 0.1-about 2.0%, desirably about0.1-about 1.0%, and optimally about 0.2-about 0.4%, by weight, ofanother IUPAC Group 13 element, such as aluminum, calculated on anelemental basis based on the molecular sieve.

The porous microcrystalline material of the isomerization catalystpreferably is composited with a binder. The proportion of binder in thecatalyst is about 5-about 90%, preferably about 10-about 70%, andoptimally about 50%, by weight. The remainder can be metal and othercomponents discussed herein. Typically, the catalyst can contain about30-about 90%, preferably about 50%, by weight, of the aluminosilicatezeolite.

Usually catalyst particles are homogeneous with no concentrationgradients of the catalyst components. Alternatively, the catalystparticles may be layered, for example, with an outer layer of a boundzeolite bonded to a relatively inert core. Examples of layered catalystscan be found in U.S. Pat. No. 6,376,730 B1 and U.S. Pat. No. 4,283,583.

The binder should be a porous, adsorptive material having a surface areaof about 25-about 500 m²/g that is relatively refractory to conditionsutilized in a hydrocarbon conversion process. Typically, the binder caninclude (1) a refractory inorganic oxide such as an alumina, a titania,a zirconia, a chromia, a zinc oxide, a magnesia, a thoria, a boria, asilica-alumina, a silica-magnesia, a chromia-alumina, an alumina-boria,or a silica-zirconia; (2) a ceramic, a porcelain, or a bauxite; (3) asilica or silica gel, a silicon carbide, a synthetically prepared ornaturally occurring clay or silicate, optionally acid treated, as anexample, an attapulgite clay, a diatomaceous earth, a fuller's earth, akaolin, or a kieselguhr; (4) a crystalline zeolitic aluminosilicate,either naturally occurring or synthetically prepared, such as FAU, MEL,MFI, MOR, MTW (IUPAC Commission on Zeolite Nomenclature), in hydrogenform or in a form that has been exchanged with metal cations, (5) aspinel, such as MgAl₂O₄, FeAl₂O₄, ZnAl₂O₄, CaAl₂O₄, or a compound havinga formula MO—Al₂O₃ where M is a metal having a valence of 2; or (6) acombination of two or more of these groups.

A preferred refractory inorganic oxide for use as a binder is analumina. A suitable alumina material is a crystalline alumina known as agamma-, an eta-, and a theta-alumina, with a gamma- or an eta-aluminabeing preferred.

The catalyst may contain a halogen component, including either fluorine,chlorine, bromine, iodine or a mixture thereof, with chlorine beingpreferred. Desirably, however, the catalyst contains no added halogenother than that associated with other catalyst components.

One shape for the support or catalyst can be an extrudate. Generally,the extrusion initially involves mixing of the zeolite with optionallythe binder and a suitable peptizing agent to form a homogeneous dough orthick paste having the correct moisture content to allow for theformation of extrudates with acceptable integrity to withstand directcalcination. Extrudability may be determined from an analysis of themoisture content of the dough, with a moisture content in the range ofabout 30-about 70%, by weight, being preferred. The dough may then beextruded through a die pierced with multiple holes and thespaghetti-shaped extrudate can be cut to form particles in accordancewith known techniques. A multitude of different extrudate shapes ispossible, including a cylinder, a cloverleaf, a dumbbell, a trilobe, ora symmetrical or an asymmetrical polylobate. Furthermore, the dough orextrudate may be shaped to any desired form, such as a sphere, by, e.g.,marumerization that can entail one or more moving plates or compressingthe dough or extrudate into molds.

Alternatively, support or catalyst pellets can be formed into sphericalparticles by accretion methods. Such a method can entail adding liquidto a powder mixture of a zeolite and binder in a rotating pan or conicalvessel having a rotating auger.

Generally, preparation of alumina-bound spheres involves dropping amixture of molecular sieve, also, and gelling agent into an oil bathmaintained at elevated temperatures. Examples of gelling agents that maybe used in this process include hexamethylene tetraamine, urea, andmixtures thereof. The gelling agents can release ammonia at the elevatedtemperatures that sets or converts the hydrosol spheres into hydrogelspheres. The spheres may be then withdrawn from the oil bath andtypically subjected to specific aging treatments in oil and an ammoniasolution to further improve their physical characteristics. Oneexemplary oil dropping method is disclosed in U.S. Pat. No. 2,620,314.

Preferably, the resulting supports are then washed and dried at arelatively low temperature of about 50-about 200° C. and subjected to acalcination procedure at a temperature of about 450-about 700° C. for aperiod of about 1-about 20 hours.

Optionally, the catalyst is subjected to steaming to tailor its acidactivity. The steaming may be effected at any stage of the zeolitetreatment. Steaming conditions can include a water concentration ofabout 5-about 100%, by volume, pressure of about 100 kPa-about 2 MPa,and a temperature of about 600-about 1200° C. Preferably, the steamingtemperature is about 650-about 1000° C., more preferably at least about750° C., and optimally may be at least about 775° C. In some cases,temperatures of about 800-at least about 850° C. may be employed. Thesteaming should be carried out for a period of at least one hour, andperiods of about 6-about 48 hours are preferred. Alternatively or inaddition to the steaming, the composite may be washed with one or moresolutions of an ammonium nitrate, a mineral acid, or water. The washingmay be effected at any stage of the preparation, and two or more stagesof washing may be employed. The catalyst can contain at least about 30%,preferably about 30-about 50%, by weight, silicon, calculated on anelemental basis based on the catalyst.

The alkylaromatic feed mixture contacts the isomerization catalyst inthe liquid phase at suitable first isomerization conditions. Suchconditions can include a temperature ranging from about 200-about 600°C., preferably from about 250-about 350° C. Generally, the pressure issufficient to maintain the feed mixture in liquid phase, generally fromabout 500 kPa-about 5 MPa. The first isomerization stage 220 can containa sufficient volume of catalyst to provide a liquid hourly spacevelocity with respect to the feed mixture of about 0.5-about 50 hr⁻¹,preferably about 0.5-about 20 hr⁻¹.

At least part of an intermediate stream in a line 250 is contacted inthe second isomerization stage 260. The second isomerization stage 260can include at least one reactor 290 having a second isomerizationcatalyst. At least a part of a stream rich in at least one naphthene,preferably a C8 naphthene, can be fed to the reactor 290 as well. Thisstream can be provided in a line 310 to the reactor 290 or combined withthe intermediate stream in the line 250 and fed to the reactor 290.Thus, the intermediate stream and the stream rich in at least onenaphthene can be a combined feed to the second isomerization stage 260.The stream rich in at least one naphthene will be described in greaterdetail hereinafter.

Desirably, the intermediate stream and/or stream rich in at least onenaphthene are preheated by one or more suitable exchangers and/orheaters in the presence of a hydrogen-rich gas to the desired reactiontemperature and then passed into the reactor 290. The ethylbenzeneconversion catalyst can include a microcrystalline material. Such amicrocrystalline material can include one or more of BEA, MTW, FAU,MCM-22, UZM-8, MOR, FER, MFI, MEL, MTT, Omega, UZM-5, TON, EUO, OFF,NU-87 and MgAPSO-31.

An exemplary zeolitic molecular-sieve component of the secondisomerization catalyst can be MTW, also characterized as ZSM-12.Alternatively, the exemplary ethylbenzene catalyst can be one or more ofthe ATO framework types according to the ATLAS OF ZEOLITE STRUCTURETYPES, such as the MgAPSO-31 molecular sieve as disclosed in U.S. Pat.No. 4,758,419.

The intermediate stream and/or the stream rich in naphthenes can contactthe ethylbenzene conversion catalyst in the presence of hydrogen atsuitable conditions. Typically, such conditions include a temperature ofabout 200-at least about 600° C., preferably about 300-about 500° C.Generally, the pressure is about 100 kPa-about 5 MPa, preferably lessthan about 3 MPa. The second isomerization stage 260 can contain asufficient volume of catalyst to provide a liquid hourly space velocitywith respect to the combined streams of about 0.5-about 50 hr⁻¹,preferably about 0.5-about 20 hr⁻¹. The intermediate stream optimally isreacted in admixture with hydrogen at a hydrogen/hydrocarbon mole ratioof about 0.5:1-about 25:1. Other inert diluents, such as nitrogen,argon, and light hydrocarbons, may be present. Exemplary conditions andcatalyst for the second isomerization stage 260 are disclosed in US2007/0004947.

The isomerized product from the second isomerization stage 260 caninclude a concentration of at least one alkylaromatic isomer that ishigher than the equilibrium concentration at the first or secondisomerization conditions. Desirably, the isomerized product in a line280 is a mixture of one or more C8 aromatics having a concentration ofpara-xylene that is higher than the equilibrium concentration at thefirst or second isomerization conditions. The concentration ofpara-xylene can be at least about 24.2%, often at least about 24.4%, andmay be at least about 25%, by weight. The C8 aromatic ring loss relativeto the feed mixture (defined hereinafter) in the line 104 is usuallyless than about 2.0%, preferably less than about 1.5%.

The isomerized product in the line 280 can be fed to a separation zone300. The separation zone 300 can be one or more distillation towers,solvent extractors, and/or mol sieve separators. An exemplary separationzone utilizing fractionation is disclosed in U.S. Pat. No. 3,835,198.The isomerized product is sent to the separation zone 300 to obtain alighter stream containing naphthenes and a heavier stream in a line 320.Typically the heavier stream can contain xylenes and ethylbenzene. Inmany aromatic production facilities, this stream can be recycled back tothe xylene isomer separation zone 100 after further processing, such asfractionation. The lighter stream can contain C8 naphthenes, benzene,and toluene. The benzene and toluene can be withdrawn in a line 330 as avapor phase stream from an overhead receiver from a column in theseparation zone 300, as disclosed in U.S. Pat. No. 4,783,568. Thenaphthenes can be sent in the line 310 to the second isomerization stage260. The boiling point of the stream rich in at least one naphthene canbe about 110-about 130° C.

The stream rich in at least one naphthene can be sent via the line 310to the line 250 or the second stage 260, as described above. Thisdisposition bypasses the first isomerization stage 220, improving theisomerization conversion of that stage 220. Although not wanting to bebound by theory, it is believed that the disposition of the stream richin at least one naphthene aids in reducing C8RL by avoiding isomerizingthe naphthenes primarily in the first stage 220. Such reactions candestroy C8 ringed-compounds that may help generate more xylenes. Thesebenefits are generally unexpected due to the unpredictability of suchcatalyzed reactions.

Referring to FIG. 2, another exemplary aromatic production facility 40is depicted. The aromatic production facility 40 can include the xyleneisomer separation zone 100, the C8 isomerization zone 200, and theseparation zone 300, as discussed above. In this embodiment, theseparation zone 300 can be placed before the C8 isomerization zone 200instead of after. Thus, the non-equilibrium alkylaromatic feed mixturein the line 104 can be fed to the separation zone 300. The stream richin at least one naphthene can be taken as, e.g., an overhead from afractionation tower in the separation zone 300, with lighter compoundssuch as benzene and toluene withdrawn in the line 330. This stream canbe combined with the intermediate stream in the line 250 or sent via theline 310 to the reactor 290 in the second isomerization stage 260. Astream rich in xylenes and ethylbenzene can exit the separation zone 300through the line 320 to the C8 isomerization zone 200. This aromaticproduction facility 40 can achieve similar results as the aromaticproduction facility 20.

The elemental analysis of the catalyst components can be determined byInductively Coupled Plasma (ICP) analysis. Some components, such asmetals, can be measured by UOP Method 873-86 and other components, suchas zeolite or binder where each may contain silica, or silicon can bemeasured by UOP Method 961-98.

All the UOP methods, such as UOP-873-86 and UOP-961-98 discussed herein,can be obtained through ASTM International, 100 Barr Harbor Drive, WestConshohocken, Pa., USA.

Illustrative Embodiments

The following examples are intended to further illustrate the subjectprocess. These illustrations of embodiments of the invention are notmeant to limit the claims of this invention to the particular details ofthese examples. These examples are based on engineering calculations andactual operating experience with similar processes.

Catalyst A

A gallium-aluminum substituted zeolite catalyst is prepared by making afirst solution of 336.9 grams of NaOH with 989.6 grams of water. Asecond solution is prepared by combining 5201.1 grams of a silicasource, such as a silica source sold under the trade designation ofLUDOX AS-40 by E. I. Du Pont De Nemours and Company corporation ofWilmington, Del., with 5607.5 grams of water and mixing. During mixingof the second solution, 842.5 grams of an organic template, such astetrapropylammonium bromide (50%, by weight, water solution), is added,and then the first solution is added to the second solution followed by38.0 grams of sodium aluminate solution (LSA) and 983.6 grams ofGa(NO₃)₃ solution (8.54%, by weight, Ga₂O₃). The mixing of the combinedsolutions is continued until the mixture thickens and then thins to agel. Afterwards, the gel is transferred to an autoclave and reacted forabout 72 hours at a temperature of about 120-about 131° C. The solidzeolite is separated by a centrifuge and is washed 3 times.

The zeolite obtained from the autoclave is calcined in nitrogen for 2hours and air for 10 hours at a temperature of about 560° C. Aftercalcination, the zeolite is ammonium cation exchanged with 1.5 M NH₄NO₃solution at about 75° C. The obtained zeolite is filtered, and ammoniumcation exchanged again with the 1.5 M NH₄NO₃ solution at about 75° C.Afterwards, the zeolite is dried at 100° C. for about 12 hours to yielda zeolite containing about 3.1%, by weight, gallium and about 0.25%, byweight, aluminum based on the zeolite.

Next, about 50%, by weight, of Al₂O₃ and about 50%, by weight, of anGa—Al-MFI zeolite are composited to form oil-dropped spheres. Thespheres are dried and calcined to obtain Catalyst A.

Catalyst B

A zeolite is prepared the same as Example 1. However, about 50%, byweight, of the Al₂O₃ and about 50%, by weight, of an Ga—Al MFI zeoliteare extruded to form trilobe extrudates, which are labeled Catalyst B.

Performance

The catalysts discussed above are placed in a pilot plant flow reactor.The reactor processes two non-equilibrium C8 aromatic feeds having thefollowing approximate compositions:

TABLE 1 Feed Composition (Comparison) Component Percent, By WeightEthylbenzene 14 Para-xylene 1 Meta-xylene 56 Ortho-xylene 22 Toluene 1C8 naphthenes 6

TABLE 2 Feed Composition Component Percent, By Weight Ethylbenzene 15Para-xylene 1 Meta-xylene 60 Ortho-xylene 23 Toluene 1This feed in a liquid phase is contacted with each catalyst depictedbelow at a pressure of about 3500 kPa and a temperature of about 300° C.with no hydrogen.

The C8 ring loss or C8RL is in mole percent and defined as:(1−(C8 naphthenes and aromatics in product)/(C8 naphthenes and aromaticsin feed))*100which represents a loss of one or more C8 rings that can be convertedinto a desired C8 aromatic, such as para-xylene. This loss of feedgenerally requires more feed to be provided to generate a given amountof product, reducing the profitability of the unit. Generally, a lowamount of C8RL is a favorable feature for a catalyst. The C8RL can bemeasured in the table below at a conversion of the following formula:pX/X=[pX/(pX+mX+oX)]*100%where:pX represents moles of para-xylene in the product;mX represents moles of meta-xylene in the product;oX represents moles of ortho-xylene in the product; andX represents moles of xylene in the product.

Thus, the C8RL and a weight hourly space velocity (may be referred to asWHSV) in the table below are determined at pX/X of 23% in a productstream.

TABLE 3 Feed Catalyst A Catalyst B WHSV C8RL WHSV C8RL Table 1 15 1 90.8 Table 2 18 0.6 10.5 0.45

As depicted above, a feed having an absence of C8 naphthenes reducesC8RL by almost by 50%. Generally, such a benefit is unexpected in viewof the unpredictability of catalyzed isomerization reactions.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The preceding preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing, all temperatures are set forth uncorrected in degreesCelsius and, all parts and percentages are by weight, unless otherwiseindicated.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. An aromatic production facility, comprising: a C8 aromaticisomerization zone receiving a non-equilibrium alkylaromatic feedmixture and receiving a stream rich in at least one naphthene, the C8aromatic isomerization zone comprising: a) a first isomerization stagewherein at least a portion of the non-equilibrium alkylaromatic feedmixture is contacted at a first isomerization condition in a liquidphase in the substantial absence of hydrogen to obtain an intermediatestream; and b) a second isomerization stage wherein at least a portionof the intermediate stream and at least a portion of the stream rich inat least one naphthene are contacted at a second isomerization conditionto obtain a second isomerization stage effluent with a concentration ofat least one alkylaromatic isomer that is higher than a concentration ofthat at least one alkylaromatic isomer in the non-equilibriumalkylaromatic feed mixture; c) a napthene rich stream providingseparation zone providing at least part of the stream rich in at leastone napthene; and d) a xylene isomer separation zone providing anon-equilibrium alkylaromatic stream wherein the napthene rich streamproviding separation zone receives at least a portion of thenon-equilibrium alkylaromatic stream to provide at least part of thestream rich in at least one naphthene and at least part of thenon-equilibrium alkylaromatic feed mixture.